EMC FC5300 User Manual download pdf (Page 36)

2-6

EMC Fibre Channel Storage-System Configuration Planning

RAID Types and Tradeoffs

The following figure shows user and parity data with a data block

size of 2 Kbytes in a RAID 3 Group. Notice that the byte addresses

proceed from the first disk to the second, third, and fourth, then the

first, and so on.

Figure 2-3 RAID 3 Group

RAID 3 differs from RAID 5 in several important ways. First, in a

RAID 3 Group the hardware processes disk requests serially; whereas

in a RAID 5 Group the hardware can interleave disk requests. Second,

with a RAID 3 Group, the parity information is stored on one disk;

with a RAID 5 Group, it is stored on all disks. Finally, with a RAID 3

Group, the I/O occurs in small units (one sector) to each disk. A

RAID 3 Group works well for single-task applications that use I/Os

of blocks larger than 64 Kbytes.

Each RAID 3 Group requires some dedicated SP memory (6 Mbytes

recommended per group). This memory is allocated when you create

the group and becomes unavailable for storage-system caching. For

top performance, we suggest that you do not use RAID 3 Groups

with RAID 5, RAID 1/0, or RAID 0 Groups, since SP processing

power and memory are best devoted to the RAID 3 Groups. RAID 1

mirrored pairs and individual units require less SP processing power,

and therefore work well with RAID 3 Groups.

EMC1816

Second disk

Third disk

Fourth disk

Fifth disk

Stripe

element

size

Stripe

…

First disk

User data

Parity data

2048-2559 4096-4607 6144-6655 8192-8603

0-511

512-1023 2560-3071 4608-5119 6656-7167 8604-9115

5120-56311024-1535 3072-3583 7168-7679 9116-9627

3584-40951536-2047 5632-6143 7680-81919628-10139

Parity Parity Parity Parity Parity

Data block

Bytes

1 2 ... 31 32 33 34 35 36 37 38 39 40 41 ... 133 134

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